1. Field of the Invention
The present invention relates to a tracking control apparatus for an optical information reproducing system, and more particularly, to a tracking control apparatus for an optical information reproducing system which can perform the tracking control correction regardless of the eccentricity of an optical recording carrier such as a rotary disc.
2. Description of the Prior Art
A prior art tracking control apparatus for an optical information reproducing system will be now explained with reference to FIG. 1. In the prior art, as shown in FIG. 1, a laser beam from a laser light source (not shown) is introduced through a deflection beam splitter 1 to an objective lens 2. A beam from the objective lens 2 is converged on an optical recording carrier, such as a rotary plate or disc 3, as a reference beam. The reproduced or reflected beam from the rotary disc 3 is reversed through the objective lens 2 and transmitted to the beam splitter 1, where it is reflected by the beam splitter 1 to a light receiving element or photo-detector 4.
On the signal record surface of the optical recording rotary disc 3, an information signal is recorded on disc 3 in a coaxial or spiral (nearly coaxial) optical record track. The information signal can be a PCM (pulse code modulated) video signal or the like. For example, in one prior art system, the optical record track is formed of pit trains.
As shown in FIGS. 1 and 2, the light receiving element or photo-detector 4 comprises first and second light receiving members 4a and 4b which are divided into two members by a boundary line L. The light receiving element 4 is arranged so that the tangential direction of the record track on the rotary disc 3 corresponds to the boundary line L. First and second light receiving elements 4a and 4b generate signals from which the difference between the envelopes of the respective signals is formed and used as a tracking error signal. The respective output signals from the light receiving members 4a and 4b are fed to a differential amplifier 5 functioning as a subtracter or comparator, through an envelope detecting circuit, for example, low pass filter 7. The tracking error signal delivered from the differential amplifier 5 is supplied to a tracking control device or means 6. Tracking control device 6 is an electro-mechanical transducer device which drives the objective lens 2 or tracking mirror (not shown) in response to the tracking error signal to carry out the tracking correction.
In FIG. 2, reference letter B designates a spot of the beam reflected on the light receiving element 4. Beam spot B comprises a non-diffraction beam area B.sub.0 and + and - primary diffraction beam areas B.sub.+1 and B.sub.-1 on the peripheral portion of the non-diffraction area B.sub.0. When the tracking of the reference beam is centered, the distribution of the diffracted beams of the reproduced beam is symmetrical on both sides of a line tangent to the record track and, the tracking error signal from the differential amplifier 5 is zero. When there is a tracking displacement or deviation in the reference beam, the distribution of the diffracted beams becomes asymmetrical. A tracking error signal, which has the level and polarity in response to the amount and polarity of the tracking displacement or deviation, is derived from the differential amplifier 5.
The above prior art tracking control apparatus has problems when the record track is not perfectly centered on disc 3, i.e., is eccentric. When a tracking deviation exists, the tracking control device 6 generates the tracking correction for the reference beam. The beam spot B of the reproduced beam on the light receiving element 4, however, is moved in a vertical direction with respect to the boundary line L. As a result, the output signal from the light receiving member, to which the beam spot B moves, becomes large and functions as an external disturbance for the tracking error signal, thereby making the tracking control operation unstable. This external disturbance becomes large as the eccentricity (.+-.100.mu.) of the optical recording carrier or rotary disc 3 is large.
The external disturbance of the tracking error signal will be further described with reference to FIG. 3. FIG. 3A shows the waveform of a tracking error signal when the tracking control device 6 is not supplied with the tracking error signal. Since the dynamic range of the tracking error signal corresponds to 1/2 of the track pitch, the dynamic range of the tracking error signal is accordingly very small. When an AC current, as shown in FIG. 3C, is applied to the tracking control device 6 to move the reference beam in a direction perpendicular to a line tangent to the record track, the tracking error signal of FIG. 3A is amplitude modulated by the AC current illustrated in FIG. 3C to form the signal shown in FIG. 3B. The amount of displacement generated by the tracking control device 6 is proportional to the amount of eccentricity of the rotary disc 3 in its radial direction, so that when the undulation of the tracking error signal exceeds the value corresponding to 1/4 of the track pitch, no tracking control correction is applied.